Multilevel converters are known in the art. For transportation purposes, for instance, there is a need for transformation of a medium input AC (alternating current) voltage to a medium or low output DC voltage. In general, an AC/DC converter can be implemented in a two-stage approach, including an active front end stage and a DC/DC converter stage.
A control unit is provided to control the active front end stage and the DC/DC converter stage in such a way to provide a constant average output DC link voltage. A number of sensing elements are used for measuring the line input voltage, line input current, DC link output voltage, resonant current on the primary/secondary side of the DC/DC converter, and the DC link voltages of all levels on the active front end units.
This kind of multilevel converter has a multilevel topology, where the active front end stages are serially connected, while the isolated DC/DC converter stages are paralleled at the output. In case of a multilevel converter topology, the sensing elements have to measure the current on the secondary sides of each DC/DC converter and the DC link output voltages of each active front end stage.
The control unit serves for controlling the active front end stage and the DC/DC converter stage such that the key control targets of an average constant output DC link voltage and of controlling the power factor (e.g. near unity) and the total harmonic distortion (THD) of the line current as low as possible are met. Since the multilevel converter in medium voltage applications is operated directly from a medium voltage source, the isolation requirements for the sensing equipment are quite demanding. Hence, there can be a reduction in the number of voltage/current sensors for measurement needed to apply the control scheme in the control unit.
EP 2 180 586 A1 and U.S. Pat. No. 6,344,979 B1 disclose an AC/DC converter which has the above-described two-stage topology and uses an LLC resonant circuit or a CLL resonant circuit, respectively, to make the DC/DC converter a resonant converter.
DE 198 27 872 A1 discloses a multilevel power electronics transformer having a two-stage topology with an active front end unit and a non-resonant DC/DC converter.
DE 19 750 041 C1 discloses a DC/DC converter having an active front end and a DC/DC converting stage being a resonant stage.
Furthermore, U.S. Pat. No. 6,218 792 B1 discloses a modular converter arrangement with a focus on modules, mechanical features and connectivity.
U.S. Pat. No. 5,646,835 discloses a series resonant circuit which includes an inverter having Insulated Gate-Bipolar Transistors (IGBTs). The series resonant circuit includes a controller which utilizes phase and frequency modulation in conjunction with a logarithmic amplifier to control the inverter.
U.S. 2006/0221653 A1 discloses a multilevel converter-based intelligent universal transformer including back-to-back interconnected multilevel converters coupled to a switched inverter circuit via a high-frequency transformer. The input of the universal transformer can be coupled to a high voltage distribution system, and the output of the universal transformer can be coupled to a low voltage application.
In document T. Zhao, G. Wang, J. Zeng, S. Dutta, S. Bhattacharya and A. Q. Huang, “Voltage and Power Balance Control For a Cascaded Multilevel Solid-State Transformer”, IEEE Applied Power Electronics Conference and Exposition, APEC 2010, pp. 761-767, a dual active bridge converter is used for a DC/DC converter stage. Power flow in the dual active bridge converter is controlled by controlling the phase shift between the voltages applied on the input and the output side across a transformer leakage inductance. To achieve power balancing among the stages, all voltages and currents on each level on both sides need to be measured.
A converter of the generic type is also disclosed in U.S. Pat. No. 5,233,509.